30+ day terms were relevant when postage took 3-5 days and it took 5 business days for a standard cheque to clear. In this day and age when payment can be made very easily and quickly and in an automated fashion I see no reason for terms to exceed 14 days. You've done the work, the client presumably has the funds available. Why on earth shouldn't you be paid within a week?

FYI my proper professional services clients have no issues with 14 day payment terms - one paid late once, I sent them a reminder and they've always paid on time since for any other ad-hoc or project work I've completed for them. B2B - no agencies.

If you google "Accountancy Invoicing Best Practice" it appears most businesses are very accepting of 14 day terms, with some places suggesting that even 7 is becoming more widely accepted.

Why is it acceptable to give a client 30 day terms and suffer the exposure?

You're living in cloud-cuckoo land mate, it's not a matter of what's acceptable, it's business, get money in as early as possible, pay money out as late possible.

Doesn't really apply to contractors though, since the out of the agents fees come some level of factoring.